While some metro metro communities are blaming aging infrastructure for the discharge of raw sewage into area waters after heavy rains last weekend, the Hennepin County community with the oldest sewers didn’t spill a drop of sewage.

In fact, Minneapolis has had only two small sewage overflows since 2006.  Those 2010 discharges of 200,000 gallons pale in comparison to the 360 million gallons spilled in 1984.

The city’s stellar record is the result of a long and costly investment in separating its stormwater and sanitary sewers. The job isn’t done, but most of the heavy lifting was completed between 1960 and 1995.

“We’ve made amazing progress in separating our sewers, even relative nationwide to comparable cities,” said Kelly Moriarty, an engineering supervisor for the city.

Mound and the Metro Council were blaming each other in the wake of last weekend's rains for the pumping of untreated sewage into Lake Minnetonka.

In the old days, both sewers that carried stormwater and those carrying sewage emptied into the Mississippi River and other natural waters.  By the late 1930s, household and business sewage headed to the new Pigs Eye metro sewage plant.

New developments got dual piping to handle the two flows. But that left hundreds of miles of older streets where sewers still had combined roles. It wasn’t until a massive Minneapolis street reconstruction program that began in the 1960s that those older streets got separate storm drains. The city also worked to take out connections where sewage can cross from one pipe to another.  The separation accelerated after 1986, both under pressure from environmental regulators and because state and federal aid supplemented utility bills paid by customers. To preserve capacity, property owners also were required to disconnect downspouts from sewers, and property owners have been encourage to adopt practices to hold back rain flow..

Overflows can happen when it rains or there’s heavy snowmelt because excess water reaching waste sewers flows into storm sewers that head directly to the Mississippi.  When sewage pipes reach capacity, regulators divert waste flow into the river. Otherwise pipes would burst from pressure or sewage would back up into basements.

The job of separating sewers in Minneapolis is 95 percent done, but separating remaining links has gotten harder and the remaining fixes are the most expensive. However, that degree of separation is sufficient to eliminate all but rare overflows.  

(Photo: Workers for the city work at finding breaches and repairing the city's storm tunnels. Photos by Richard Sennott.)